When building a raid array I always plan for a certain disk size and use same size for an array. It's just cleaner. I try to stick to same make/model too. When I expand in the future I try to get the same make/model or something faster.
Currently I have two raid 10's and a raid 5. The raid 5 uses 1TB drives so I will eventually retire it and probably make another raid 10 with like 4TB drives or something. Or might just use the slots to expand the other two raids. One of the raid 10's only has 4 3TB drives so that's a good candidate for expansion. Though last time I tried expanding a raid 10 it would not let me but there was something messed up with that array. in fact, I STILL can't kill that array, there's 0 drives in it and it still wont let me take it offline. Kind of a pain. I had tried to make a degraded raid 10 with 7 drives, figuring it would make a 8 drive array with one shown as removed, but instead it actually made a raid 10 with 7 drives, somehow. So a note for next time, always make raid 10's in pairs, as it wont actually treat 1 drive as missing like you'd think.